MQTT_PublishQs0(P_TOPIC_NAME, string_buf, strlen(string_buf))

这行代码是使用MQTT协议发布一个QoS为0的消息，其中：

- P_TOPIC_NAME是消息的主题名称，用于标识消息的类型和内容。
- string_buf是消息的内容，以字符串形式表示。
- strlen(string_buf)是消息内容的长度。

QoS（Quality of Service）指的是消息的传输质量，0表示最多发送一次，不保证可靠性。该函数的作用是将消息发送给MQTT服务器，由服务器进行转发给订阅该主题的设备或应用程序。

相关问题

51单片机 mqtt 代码实现

51单片机是一种常见的嵌入式系统开发平台，可以通过添加网络模块来实现MQTT协议的通信。下面是一个基本的51单片机MQTT代码实现示例：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include "mqtt.h"
#include "eth.h"
#include "lcd.h"
#include "key.h"

#define MQTT_SERVER "mqtt.example.com"
#define MQTT_PORT 1883
#define MQTT_USERNAME "username"
#define MQTT_PASSWORD "password"
#define MQTT_CLIENT_ID "client_id"

#define MQTT_TOPIC "topic"

unsigned char mqtt_buf[MQTT_BUF_LEN];
unsigned char mqtt_readbuf[MQTT_BUF_LEN];

unsigned char mqtt_connect(void)
{
    unsigned char rc = 0;

    MQTTClient("MQTT", 256, mqtt_buf, MQTT_BUF_LEN, mqtt_readbuf, MQTT_BUF_LEN);

    if (MQTTConnect(MQTT_CLIENT_ID, MQTT_USERNAME, MQTT_PASSWORD) != 0) {
        rc = 1;
    }
    return rc;
}

unsigned char mqtt_publish(char *msg)
{
    unsigned char rc = 0;

    if (MQTTConnect(MQTT_CLIENT_ID, MQTT_USERNAME, MQTT_PASSWORD) != 0) {
        rc = 1;
    }
    else {
        MQTTMessage msg;
        msg.qos = QOS0;
        msg.retained = 0;
        msg.payload = msg;
        msg.payloadlen = strlen(msg);
        if (MQTTPublish(MQTT_TOPIC, &msg) != 0) {
            rc = 1;
        }
        MQTTDisconnect();
    }
    return rc;
}

void main(void)
{
    unsigned char rc;
    unsigned char data;

    lcd_init();
    eth_init();
    mqtt_connect();

    while (1) {
        data = key_scan();
        if (data != 0) {
            char msg[16];
            sprintf(msg, "Key pressed: %c", data);
            mqtt_publish(msg);
        }
    }
}

这个例子使用了MQTT库和以太网库来连接MQTT服务器，并使用按键来发布消息。在使用之前需要根据实际情况修改MQTT服务器地址、端口、用户名、密码、客户端ID和主题等参数。

STC89C52 ESP-01 MQTT固件 订阅消息 C语言代码

以下是基于STC89C52单片机和ESP-01 WiFi模块的MQTT订阅消息的C语言代码示例：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stc89c52.h>

#define MQTT_MAX_PACKET_SIZE    1024
#define MQTT_PORT               1883

char mqtt_buf[MQTT_MAX_PACKET_SIZE];
unsigned short mqtt_next_packet_id = 1;

/**
 * 将一个16位整数转换成字节流（大端序）
 */
void mqtt_write_uint16(unsigned char *buf, unsigned short value) {
    buf[0] = (unsigned char)(value >> 8);
    buf[1] = (unsigned char)(value & 0xFF);
}

/**
 * MQTT CONNECT消息
 */
int mqtt_connect(char *client_id, char *username, char *password) {
    unsigned char header[] = {
        0x10, // MQTT Control Packet type = CONNECT (0x10)
        0x00, // Remaining Length
        0x00, // Protocol Name Length MSB
        0x04, // Protocol Name Length LSB
        'M', 'Q', 'T', 'T', // Protocol Name = MQTT
        0x04, // Protocol Version = 4
        0x00, // Connect Flags
        0x00, 0x3C // Keep Alive = 60 seconds
    };
    unsigned char *payload = (unsigned char *)malloc(2 + strlen(client_id) + strlen(username) + strlen(password));
    unsigned short payload_len = 2 + strlen(client_id) + strlen(username) + strlen(password);
    unsigned char *ptr = payload;
    
    mqtt_write_uint16(ptr, strlen(client_id));
    ptr += 2;
    memcpy(ptr, client_id, strlen(client_id));
    ptr += strlen(client_id);
    
    if (username != NULL) {
        payload_len += (2 + strlen(username));
        *ptr++ = 0x00;
        *ptr++ = strlen(username);
        memcpy(ptr, username, strlen(username));
        ptr += strlen(username);
    }
    
    if (password != NULL) {
        payload_len += (2 + strlen(password));
        *ptr++ = 0x00;
        *ptr++ = strlen(password);
        memcpy(ptr, password, strlen(password));
        ptr += strlen(password);
    }
    
    mqtt_write_uint16(header + 1, sizeof(header) + payload_len - 1);
    unsigned short packet_len = sizeof(header) + payload_len;
    memcpy(mqtt_buf, header, sizeof(header));
    memcpy(mqtt_buf + sizeof(header), payload, payload_len);
    free(payload);
    
    return packet_len;
}

/**
 * MQTT SUBSCRIBE消息
 */
int mqtt_subscribe(char *topic, unsigned char qos) {
    unsigned char header[] = {
        0x82, // MQTT Control Packet type = SUBSCRIBE (0x82)
        0x00, // Remaining Length
        0x00, mqtt_next_packet_id // Packet Identifier MSB/LSB
    };
    unsigned char *payload = (unsigned char *)malloc(2 + strlen(topic) + 1);
    unsigned short payload_len = 2 + strlen(topic) + 1;
    unsigned char *ptr = payload;
    
    mqtt_write_uint16(ptr, mqtt_next_packet_id++);
    ptr += 2;
    strcpy(ptr, topic);
    ptr += strlen(topic);
    *ptr++ = qos;
    
    mqtt_write_uint16(header + 1, sizeof(header) + payload_len - 1);
    unsigned short packet_len = sizeof(header) + payload_len;
    memcpy(mqtt_buf, header, sizeof(header));
    memcpy(mqtt_buf + sizeof(header), payload, payload_len);
    free(payload);
    
    return packet_len;
}

/**
 * 从串口接收数据
 */
void serial_receive(char *buf, unsigned short len) {
    unsigned short i = 0;
    for (i = 0; i < len; i++) {
        buf[i] = SBUF;
    }
}

/**
 * 向串口发送数据
 */
void serial_send(char *buf, unsigned short len) {
    unsigned short i = 0;
    for (i = 0; i < len; i++) {
        SBUF = buf[i];
        while (!TI);
        TI = 0;
    }
}

/**
 * 延迟函数，单位为微秒
 */
void delay_us(unsigned int us) {
    unsigned char i;
    while (us--) {
        for (i = 0; i < 12; i++);
    }
}

int main() {
    char client_id[] = "my_client_id";
    char username[] = "my_username";
    char password[] = "my_password";
    char topic[] = "my_topic";
    unsigned char qos = 1;
    unsigned short packet_len = 0;
    unsigned short i = 0;
    
    TMOD = 0x20; // Timer1 Mode 2
    TH1 = 0xFD; // 波特率9600，Timer1的重载值为0xFD
    TL1 = 0xFD;
    TR1 = 1;
    SCON = 0x50; // 8位数据，无校验，1位停止位，SM0=0, SM1=1
    
    while (1) {
        packet_len = mqtt_connect(client_id, username, password);
        serial_send(mqtt_buf, packet_len);
        
        delay_us(100000); // 延时100ms
        
        packet_len = mqtt_subscribe(topic, qos);
        serial_send(mqtt_buf, packet_len);
        
        delay_us(100000); // 延时100ms
        
        if (RI) {
            // 有数据可读
            i = 0;
            while (RI && i < MQTT_MAX_PACKET_SIZE) {
                mqtt_buf[i++] = SBUF;
            }
            // 处理收到的MQTT消息
            // ...
        }
    }
    return 0;
}

需要注意的是，这段代码仅作为示例，实际使用时需要根据实际情况进行修改和优化。